The terahertz frequency range is usually defined as 0.1 to 10 THz. Terahertz waves are electromagnetic waves with frequencies higher than microwaves but lower than infrared radiation and visible light. This bandwith is known as the terahertz gap, because it proved to be difficult to produce suitable and small devices emitting electromagnetic radiation in this frequency range with enough power. Terahertz waves possess many advantages for different applications, as for example in spectroscopy, non-destructive testing of non-conductive materials such as plastics, foam, composites, ceramics, paper, wood and glass, e.g. in space industry, non-ionizing medical imaging and tumor detection, high resolution close range radar and security detection.
The radiation of a fast electron beam moving with a drift velocity v0 is known as the Smith-Purcell effect, (1 S. J. Smith and E. M. Purcell, Phys. Rev. 92, 1069(1953)). This phenomenon is used, for example, in vacuum microwave devices, but it has been never observed that solid-state structures can be used to emit terahertz radiation. Numerous attempts to build a solid-state terahertz emitter based on the same physical principle failed, see a discussion in Ref.2 (S. A. Mikhailov, Phys. Rev. B 58, 1517 (1998)).
In recent years, graphene, which is an allotrope of carbon, has become the subject of theoretical and experimental studies, also in the field of electronic nanodevices. A technology needed for the production of Smith-Purcell-type graphene-based generators and amplifiers is already available. For example, a production of graphene layers on hexagonal boron nitride (h-BN) substrates is described in Ref.3 (C. R. Dean, A. F. Young, I. Meric, C. Lee, L. Wang, S. Sorgenfrei, K. Watanabe, T. Taniguchi, P. Kim, K. L. Shepard, et al., Nature Nanotechnology (2010)). In Ref.4 (L. A. Ponomarenko, A. K. Geim, A. A. Zhukov, R. Jalil, S. V. Morozov, K. S. Novoselov, I. V. Grigorieva, E. H. Hill, V. V. Cheianov, V. I. Falko, et al., Nature Physics (2011)) a layered structure BN-graphene-BN-graphene has been studied (with non-structured graphene layers). A system of narrow stripes in a single graphene layer with the stripe width from 4 μm down to 1 μm was prepared and investigated in Ref.5 (L. Ju, B. Geng, J. Horng, C. Girit, M. Martin, Z. Hao, H. A. Bechtel, X. Liang, A. Zettl, Y. R. Shen, et al., Nature Nanotechnol. (2011), doi:10.1038/nnano.2011.146). Graphene layers of cm size have been produced in Ref.6 (K. S. Kim, Y. Zhao, H. Jang, S. Y. Lee, J. M. Kim, K. S. Kim, J.-H. Ahn, P. Kim, J.-Y. Choi, and B. H. Hong, Nature 457, 706(2009).).